The mechanical response of vascular smooth muscle to vasoactive agents is being studied in vitro and in vivo. The preparations consist of arteries and veins of intermediate size (1 to 6 mm diameter in the dog) in anesthetized dogs and in vitro. Specific current objectives are: (a) To define quantitatively the response of these vessels to agents that act through adrenergic and muscarinic receptors; (b) To study the time-dependent properties of these responses and learn how normal pulsatile distension of the vessel affects their reactivity; (c) To compare various methods of measuring response, including the relatively new approach of calculating the mechanical energy stored elastically in the vessel wall, and the energy expended in vasomotion. The behavior of smooth muscle in these intermediate vessels has not been explored systematically in vivo, because earlier investigations have concentrated on the responses of arterioles. The arterioles exert the predominant control on mean pressure and flow to the periphery, but the larger arteries control the amount of pulsation transmitted to the microcirculation, and hence influence capillary function. Information obtained in these experiments may throw light on the mechanism of some pathologic vascular states, and may be relevant to drug therapy of hypertension and other disorders.